Hide details when zooming out

[Matthias Schoettle]

From Jörg: Currently, when zooming out, although the small text is completely unreadable, it still draws, which slows down the display significantly. It would make sense to stop drawing small text once the zoom level is far out.

For example, when zooming out the structural view, the classes could just show the class name. No attributes and methods.

[Matthias Schoettle]

This might be good to be addressed for the demo, but more importantly for COMP-361, especially if the CPU issues are not resolved (see issue #43)